This invention relates generally to novel methods employing magnetic fields for controlling chemical reactions initiated by free radicals, such as emulsion polymerization, and more particularly, for improving the efficiency and yield of such reactions.
Emulsion polymerization is commonly employed to produce high molecular weight polymers at relatively rapid rates. An emulsion polymerization typically involves four fundamental ingredients: (1) an aqueous dispersion medium; (2) dispersed droplets of monomer; (3) an emulsifier such as a micelle-generating detergent; and (4) an initiator.
An explanation of the conventional mechanism of emulsion polymerization is provided by the Smith-Ewart theory (see Smith, W. V. and Ewart, R. W., J. Chem. Phys., 1948, Vol. 16, p. 592). According to this theory, there are three stages of emulsion polymerization. In the initial stage the loci of initiation of polymerization are micelles (aggregates of large numbers, e.g., 50-100, of detergent molecules) swollen with monomer molecules. During this stage, dispersed monomer droplets serve as a reservoir of monomer molecules, and "nuclei" of growing polymer particles are produced in the micelles. Toward the end of this stage (10-20% conversion of monomer), the number and size of growing polymer particles increase and the number of micelles decreases because detergent molecules become preferentially adsorbed on growing polymer particles which swell with monomer that is available from the dispersed droplet reservoir. In the second stage, the major growth of polymer occurs as the volume of monomer swollen polymer particles increases and the volume of monomer reservoir decreases. During this stage, the loci of polymerization are considered to be exclusively the polymer particles. In the final stage the monomer disappears completely and the unreacted monomer exists only in swollen polymer particles.
The Smith-Ewart theory provides a satisfactory framework for explaining the exceptional features of emulsion polymerization, i.e., the simultaneous achievement of a high molecular weight polymer and a high polymerization rate result from the generation of a large number of isolated microscopic reaction vessels (in the first stage) that allow for the uninterrupted propagation of polymerization in the absence of chain transfer or termination. Emulsion polymerizations use water-soluble initiators, which allow termination to be minimized since initiation will involve a single radical which is delivered from the aqueous phase to a monomer-swollen micelle or to a monomer-swollen polymer particle. Oil-soluble initiators are generally ineffective in emulsion polymerization, because such initiators produce pairs of radicals in the polymerization loci which recombine rapidly, thereby favoring termination before substantial polymer growth can occur.
If oil-soluble initiators (many of which are available and some of which are cheaper than water-soluble initiators) could be employed effectively for emulsion polymerization, a wider variety of polymers could be manufactured. The invention accomplishes this and other advantageous results by employing a mechanism that produces long-lived radical pairs and that substantially enhances the efficiency of escape of radicals from such radical pairs by controlling the rate at which radical combination occurs. This enables one of the radicals of a radical pair produced by an oil-soluble initiator to escape into the aqueous phase at a rate that is faster than either the rate at which the monomer is attacked or the rate at which radical combination occurs, thereby providing a situation that is analogous to that which exists with water-soluble initiators.